The present invention relates to laminated structures and is particularly concerned with green and sintered laminated structures and with a method of forming a sintered laminated structure.
When two layers of particulate ceramic or ceramic-like, eg. cermet, materials having different shrinkage rates are laminated and then sintered, there is a tendency for the edges of the laminated structure to curl away from the layer having the lesser shrinkage rate as shrinkage of the layer having the greater shrinkage rate is resisted by the layer having the lesser shrinkage rate. This is a particular problem where the curled edges are not desirable, for example when the edges of the laminated structure are intended to have seals applied to them, such as when the laminated structure forms part of a planar solid oxide fuel cell. The invention is particularly applicable to planar solid oxide fuel cells and, for convenience only, will be described hereinafter generally in relation to such laminated structures. However, it will be understood that the invention is applicable to other sintered, laminated structures.
A planar solid oxide fuel cell (SOFC) consists of a dense solid oxide electrolyte layer laminated with opposed porous electrodes. On shorting the cell through an external load, gaseous fuel oxidises at the anode resulting in release of electrons which flow through the external load and reduce oxygen at the cathode. The charge flow in the external circuit is balanced by ionic current flows within the electrolyte. Thus, at the cathode oxygen from the air or other oxidant is converted to oxygen ions which migrate through the electrolyte layer and react with the fuel at the anode/electrolyte interface. In order to permit the reactions at the respective electrode/electrolyte interfaces, the electrode materials must be porous. On the other hand, it is essential that the electrolyte material prevents the fuel gas and oxygen-containing gas from contacting each other and it must therefore be dense in order to prevent passage of the gases therethrough.
A typical solid oxide electrolyte material used in an SOFC is Y2O3-doped ZrO2, which is an oxygen ion conductor. However, many other materials have been proposed and the invention is applicable to all of these. A variety of different anode and cathode materials have been proposed for use at the fuel and air sides of SOFCs respectively, but the present invention is only concerned with such materials of a ceramic or ceramic-like nature, such as a Ni/ZrO2 cermet on the fuel side and strontium doped lanthum manganite (LSM or LaMnO3) and strontium doped praseodymium manganite (PSM) on the air side.
In one method a planar SOFC is made by forming a green ceramic or ceramic-like anode layer and firing it to at least partially sinter the anode material, applying a green solid oxide electrolyte layer to the sintered anode layer and then firing it to sinter the electrolyte material and, if necessary, the anode material. If the laminated structure has curled at the edges due to the different densities of the anode layer and electrolyte layer, weights may be applied to the curled edges before firing the laminated structure again in a process known as creep flattening. Alternatively, the weights may be applied to the edges of the structure before firing the electrolyte layer. Subsequently, a green ceramic or ceramic-like cathode layer is applied to the side of the sintered electrolyte layer remote from the anode material and sintered. The cathode layer is relatively thin, is generally sintered at a lower temperature than the electrolyte and may not extend to the edges of the electrolyte layer, so it generally does not present any problem with curling. In an alternative arrangement, rather than applying weights to the two-layer laminated structure and firing the weighted structure, the curled edges of the sintered laminated structure are ground or trimmed off to provide a substantially flat structure.
It would be highly advantageous to alleviate such curling at the edges of a fired laminated structure.